The composition of the normal gastrointestinal flora varies somewhat from individual to individual. Some bacterial species may be carried only transiently, but most are fairly permanent. Some members of the normal flora can become pathogenic if they acquire additional virulence factors (e.g., E. coli) or are introduced into normally sterile sites (e.g., Staphylococcus aureus). Normal flora is generally beneficial—for example, the normal flora may prevent pathogenic microorganisms from proliferating in the body (a phenomenon known as colonization resistance), and may also produce essential nutrients (e.g., vitamin K is produced by the gut flora).
The use of antibiotics is ubiquitous among children and adults for bacterial infections, and they are often also prescribed for viral infections. This prolific use has come under criticism for various reasons, most notably for inducing microbial resistance to previously effective antibiotics and rendering them less effective or ineffective against dangerous human pathogens. For example, multidrug-resistant strains of Mycobacterium tuberculosis seriously threaten tuberculosis (TB) control and prevention efforts. Administration of broad-spectrum antibiotics has a profound effect on the normal flora and can result in colonization with antibiotic-resistant organisms. Antibiotic-mediated disruption of the normal flora can lead to fungal infections, such as invasive candidiasis, or to antibiotic-associated colitis caused by Clostridium difficile. 
Several neurological or neuropsychiatric conditions, such as autism may have gastrointestinal etiology. Published data lend credence to the notion that an alteration in bowel microflora may be associated with autistic symptoms (Sandler et al., J. Child Neural. 15, 429-435, 2000; Finegold et al., Clin. Infect. Dis. 35 (Suppl. 1), S6-S16, 2002; Song et al., Appl. Environ. Microbial. 70, 6459-6465, 2004; Parracho et al., J. Med. Microbial. 54, 987-991, 2005; Finegold et al., Medical Hypotheses 70, 508-511).